Serbia has threatened to retaliate after neighbour Croatia halted all cargo traffic from Serbia following an influx of asylum seekers over their joint border.

More than 30,000 people, many of them Syrian refugees, have entered Croatia from Serbia since Tuesday last week, when Hungary barred their entry to the European Union over its southern border with Serbia with a metal fence.

Croatia said it cannot cope with the numbers; a new crossing point emerged overnight at the Croatian village of Bapska, with hundreds seen walking through rolling hills to a camp Croatia opened for asylum seekers while authorities organise their transport north to Hungary or west to Slovenia.

Several thousand more were waiting in no-man's land between Sid in Serbia and Tovarnik in Croatia; some spent the night sleeping in a cemetery.

Croatia closed all but one road border crossing with Serbia and early on Monday halted trucks from entering on the highway between Belgrade and Zagreb, one of Serbia's two main arteries to the west.

The blockage created a 12-kilometre queue of trucks on the Serbian side.

The Croatian government cited security reasons but local media say Zagreb was trying to put pressure on Belgrade to slow the flow of migrants over their border and send more north to Hungary or east to Romania.

Croatian interior minister Ranko Ostojic told RTL television: "I think it is perfectly clear that what we are seeing is organised transport of migrants directly to Croatia (from Serbia)."

'We will be forced to protect our country by legal means'

The Serbian government called an emergency session on Tuesday afternoon to discuss possible counter-measures.

"There is no rational, logical explanation for stopping the flow of goods, of cargo traffic, on one of the main arteries," Serbian justice minister Nikola Selakovic told state television.

"Closing that route is inflicting millions in damage to Serbian business and the state every day."

If Croatia does not lift the blockade, Serbian prime minister Aleksandar Vucic said, "due to violations of numerous bilateral and international agreements we will be forced to protect our country by legal means".

"That will hit their country far harder than their measures are hurting ours," he said.

International organisations make appeal to European nations

The Organisation for Economic Co-operation and Development (OECD) said Europe must urgently produce a co-ordinated response to the mass exodus of asylum seekers from Syria and Iraq, and whatever measures are agreed will need to be extended and beefed up as part of a wider adjustment in long-term migration policy.

"Europe has both the obligation and the capacity to deal with this unprecedented and tragic crisis," the Paris-based organisation, which offers policy advice to industrialised countries, said.

Failure to agree a joint emergency plan and couple it with a modernised long-term policy on integration of migrants could fuel the fear of foreigners that partly explains government inaction to date, the OECD said in an annual report.

The OECD alludes to voter concern about integration and failure by governments to build consensus on an issue which has in recent years played into the hands of anti-immigrant parties, like France's National Front.

The International Organisation for Migration's spokesman Leonard Doyle told the ABC's The World program from Geneva that "the idea that refugees are a burden is misplaced ... all the studies show that they tend to work harder, they pay their taxes, they're family centred and they're a huge contribution to society".

"There is always the problem that politicians [use migration to score political points], but the silent majority seem to understand that immigration is an important thing that replenishes societies".

A European Union proposal to find spaces for 120,000 refugees will not work unless reception facilities are provided for tens of thousands at any time, the UN refugee agency said.

"A relocation program alone, at this stage in the crisis, will not be enough to stabilise the situation," UNHCR spokeswoman Melissa Fleming said.

EU interior ministers will hold emergency talks on Tuesday to bridge deep divisions over Europe's worst migration crisis since World War II, as pressure builds for member states to reach an agreement.

The meeting will see ministers discuss controversial binding quotas to relocate 120,000 refugees around the EU from frontline states after they failed to reach a deal last week, and ahead of a full summit of the bloc's 28 leaders on Wednesday.